Fiber optic cable contains multiple, mutually-isolated, coated optical-fibers. The cable is flexible and, during installation, e.g., in an existing dwelling or multi-dwelling unit, aggressive bending may be required to avoid obstruction. Accordingly, each individual coated optical-fiber in the cable is subjected to the same bending.
Each mutually-isolated optical-fiber has glass at its core and cladding to form a proper boundary condition for the wave guide. The clad glass is coated with a soft coating/cushion which, in turn, is covered by a hard coating protection layer. Finally, a plurality of those optical-fibers is cabled to a needed diameter to form a useful cable.
When a fiber optic cable is bent, mechanical stresses are developed in the glass of its encapsulated fibers, in their coatings and finally in the cable, i.e., compression in the glass and coatings of a glass fiber at the inside of the bend and tensile force in that glass and coatings at the outside of the bend. The soft coating/cushion helps to reduce the stresses on the glass of the coated optical-fiber when it is bent and the hard coating prevents the coating from breaking away from the fiber cladding while also protecting the glass core/cladding and soft coating. Traditionally, fiber could not be bent below 15 mm bend radius (established by industry standard G.652-D), because below a 15 mm bend radius, light would leak out of the fiber cladding, and the result could be a very high light energy loss.
However, the new Bend Insensitive Fiber (industry standard G.657-B3) has a bend radius limit targeted at a much lower 5 mm radius. At this bend radius, fiber is under extreme stress, much greater stress than at 15 mm, and the fiber may break before observing a major insertion loss. At these small radii, the coating on the cladding may tend to fail in its protection role at locations of severe bends in the cable (i.e., bends having small bend-radii such as under 5 millimeters, per industry standard G.657-B-3). It appears that optical performance may now be surpassing coating performance in an optical fiber. In order to protect the fiber under this tight bend, some new coating technologies have been, and are being, developed. Exemplary embodiments relate to novel apparatus and methodology for determining robustness of various new coatings by subjecting those coatings to a proper fiber bend stress tolerance test technique.
A newer optical cable with improved coatings can protect the fiber from breaking, even for much smaller radii such as 5 millimeters although, as noted, tremendous compression and tensile forces are imposed on the newer glass core/cladding at such minimal bend radii. (The bigger the radius the lower the tension/compression forces, and the smaller the radius the higher the forces.) Therefore, there no longer is a radiation-leakage early warning of impending failure provided by the newer cable. Sudden, catastrophic failure, due to optical-fiber glass fracture, or the like, can be experienced by optical communication system users without any warning, to their dismay.
Therefore, there is a need to design better coated optical-fiber cables with improved coating systems to reduce the likelihood of glass fracture at locations of severe bends. The novel technique disclosed herein for testing bend fatigue and determining reliability of coated optical-fibers supports efforts for achieving new and improved coated optical-fiber designs.